This invention relates to a high density ceramic metal composite exhibiting improved mechanical properties and useful as a refractory for producing slide gate plates, nozzles, and the like. More particularly, this invention is directed to a ceramic metal composite which is derived from a specific formulation that may be fired at low temperatures to produce a ceramic metal composite of relative high density and which exhibits improved abrasion, corrosion, and oxidation resistance.
Ceramic refractories are useful as components for applications requiring good resistance to thermal shock, corrosion, and erosion when in contact with molten metal. Such components may be used in control means for regulating the flow of molten metals in molten metal transfer systems, for example, in the manufacture and handling of steel. Such uses include slide gates, sub-entry nozzles, and ladle shrouds. Slide gates are used for controlling the flow of molten metal from a ladle or tundish. Generally, slide gate systems consist of a fixed nozzle attached to or within a movable plate. The flow of molten metal is controlled by moving the movable plate to fully or partially align openings. During shutoff, the openings are misaligned. The principal advantages of the slide gate system over a conventional stopper rod system are its improved reliability to shutoff and ability to modulate molten metal flow. However, even the best of certain refractory systems, such as high alumina slide gate systems, are inadequate for certain molten metals, such as low-carbon, high manganese steel grades. These corrosive steel compositions will seriously attack the bonding media used in most slide gate refractories.
Historically, three main types of bond systems have been employed in manufacturing slide gate plates, nozzles, and similar shaped refractory articles. These three main types of bond systems are oxide bond, carbon bond, and resin bond. Conventionally-fired, oxide-bonded refractory compositions are fired in air at a temperature sufficient to cause sintering and mineralization of the fines. For alumina-based products, the primary bond phase is mullite which requires about 5-10% of silica in the composition. However, silica reacts readily with components of some steel grades resulting in rapid corrosion. Magnesia-based products exhibit very good corrosion resistance but have low thermal shock resistance and high thermal expansion which limits the use of such products to smaller articles.
Carbon-bonded refractory compositions are fired in a reducing atmosphere at a temperature sufficient to convert all hydrocarbon components to carbon and to at least partially convert silicon metal to carbide by reaction with the carbon. Carbon-bonded products generally exhibit better thermal shock resistance and better corrosion resistance than oxide-bonded products. However, the solubility of silicon carbide bond in steel contributes to corrosion, and oxidation of the carbon/carbide bond contributes to accelerated abrasion of the refractory article.
Resin-bonded refractory compositions are simply heated to a temperature sufficient to volatilize the resin solvents and polymerize the resin. Resin-bonded products contain a metallic additive, usually aluminum, which will react with the carbon to form a high strength aluminum carbide bond to replace the resin bond which decomposes during high temperature use. This high purity bond maximizes corrosion resistance. The disadvantage of resin-bonded products is the oxidation of the resin bond at low temperatures. When the aluminum metal melts, it reacts with carbon in the composition or oxygen from the air to form a strong bond and slow further oxidative degradation of the product. However, at lower temperatures, the resin bond oxidizes with little or no development of a secondary bond phase from the metal and rapid wear can result from abrasion.
Other methods for manufacturing refractory articles having high corrosion and abrasion resistance have been considered, such as the manufacture of dense ceramic products. One approach includes establishing a molten pool of aluminum covered with a thick layer of finely divided magnesium silicate particulate. Molten aluminum is transported through the particulate layer wherein it is partially oxidized by the oxidation-reduction of aluminum and magnesium silicate as well as oxidation by atmospheric oxygen. The process ultimately yields a composite of multiple oxide phases and metal phases. The reaction tends to be slow and oxidation is promoted by means of an alkali metal oxide.
There have also been attempts to produce ceramic structures more nearly approximating the net shape of the desired article by using particulate precursor metal and air oxidation. For example, particulate aluminum or aluminum alloy is combined with a metallic oxide fluxing agent, and optionally, a particulate filler refractory. The mixture is oxidized to convert the aluminum to corundum. However, a generally porous structure is developed which has intrinsically low strength properties.
Much attention has been paid and considerable efforts have been devoted to the fabrication of ceramic articles, including the manufacture of ceramic articles by in-situ oxidation of precursor metals. However, these previous attempts have been lacking in one or more respects with regard to the development of products having structural integrity rendering them useful as components for applications requiring good resistance to thermal shock, corrosion, and erosion when in contact with molten metal. For example, the migration of aluminum from a foil configuration to develop a double-walled ceramic structure severely handicaps an article manufactured in that way from adaption as a structural component for lack of strength. In addition, certain of the fabrication techniques themselves are cumbersome, requiring repetitive coatings of templates or the like interspersed with drying steps. A further deficiency of past approaches using in-situ oxidation of powders, foils, and wires to create ceramic bodies has been the exceedingly poor contact wear and erosion resistance of such bodies. It is the inherent porosity of such products which is responsible for their poor structural and wear properties and which limits the practical utility of such products.
Currently, in the United States market, the majority of the slide gate refractories are composed of ceramic-bonded, carbon-bonded, or resin-bonded high alumina or magnesia material. However, such slide gate refractories do not possess the abrasion, corrosion, or oxidation resistance necessary to withstand long preheating and long holding and teeming times. Such slide gate refractories, therefore, have a short service life in many applications. Further, the process to manufacture some of these slide gate refractories is time-consuming and costly. Therefore, it is apparent that improvements are necessary in the production of ceramic refractories for use in applications requiring good resistance to abrasion, corrosion, and oxidation.
The subject invention overcomes the above limitations and others, and teaches formulation of a ceramic metal composite having improved abrasion, corrosion, and oxidation resistance and which is cost effective.